The present invention relates to a liquid-tight seal for use in a valve primarily intended to carry liquids and particularly liquids of poor lubricating qualities such as water, solvents, fruit juices and the like.
In the past it has been common practice to use the familiar "O" ring to provide a liquid-tight seal in a valve. "O" ring seals have proved outstanding over the years in many applications but have the inherent limitation of high drag especially when used in applications where the fluid to be sealed has poor lubricating qualities. If one portion of a valve has to move in a telescoping manner within a valve body and an "O" ring is used to provide the seal, the "O" ring tends to inhibit movement by gripping the wall and tending to distort, increasing its surface area and in turn the amount of contact surface providing a frictional grip.
When an "O" ring seal is used in the valve body of a fill nozzle on a bottle-filling machine a serious problem can result. Plastic containers are very popular and because of the material costs involved the containers are continually being made thinner and lighter. Lighter container weight means less structural strength and as a result containers tend to crush as they are raised up and pressed against the filling nozzle to telescope it open. The major source of resistance to the telescoping of the fill nozzle is the "O" ring seal. The filling nozzle is normally closed with a return spring as the container lowers away when the fill process is finished. The friction caused by the "O" ring may reach 15 pounds and has to be overcome by the container in opening the valve. The return spring tension should preferably exceed the "O" ring friction to close the nozzle by at least 5 pounds, that is, a 20 pound spring. This means that the overall pressure which must be overcome by the bottle is the total of the two, that is 35 pounds. Most lightweight containers, however, cannot withstand over 20 pounds of top loading and thus may become crushed before the valve opens.
In an attempt to overcome the problems created through the use of the "O" ring seal, bellows or diaphragm type seals have been used. A bellows has equal friction in both directions; however, it must have sufficient resiliency to return the fill nozzle to the closed position after the contained is filled. The bellows or diaphragm also presents problems in that it is a complex molded part which is difficult to remove from the core during molding. The bellows relies on its inherent resiliency to obviate the need for a return spring. It has been found, however, that heat and chemicals reduce the effectiveness or the resiliency of the bellows in only a few days. It is also substantially more expensive to replace than the "O" ring and is not moldable in a wide range of materials and, therefore, has limited chemical applications.